Butadiene/sytrene block copolymers having an asymmetric structure, their preparation and their use as molding materials

Abstract

In block copolymers of the general formula A--B--A' which have a molecular weight of less than 300,000 and consist of from 18 to 40% by weight of a vinylaromatic hydrocarbon and from 60 to 82% by weight of one or more partially or completely hydrogenated hydrocarbons and in which A and A' are each a polymer block of a vinylaromatic hydrocarbon and B is a polymer block of a diene hydrocarbon, the weight ratio of the polymer blocks A to A' is less than or equal to 1:5 and greater than 1:10 and the mean molecular weight of polymer block A is from 2,000 to 20,000 and that of the polymer block A' is from 10,000 to 110,000 and the molecular weight of the B block is not less than 18,000. The block copolymers are prepared by a process in which, in a first polymerization stage, the vinylaromatic hydrocarbon is completely polymerized to form the A or A' block, in the second polymerization stage the diene hydrocarbon is completely converted to form the B block and thereafter the second portion of the vinylaromatic hydrocarbon is polymerized on to form the A or A' block, the portion of a vinylaromatic compound for the preparation of the A' block being not less than five times, and not more than 10 times, the weight used to form the A block. The block copolymers are used for the preparation of molding materials.

We claim: 
     
       1. A block copolymer of the formula A--B--A' which has a mean molecular weight of less than 300,000 and consists of from 18 to 40% by weight of a vinylaromatic hydrocarbon and from 60 to 82% by weight of one or more partially or completely hydrogenated diene hydrocarbons, wherein A and A' are each a polymer block of a styrene monomer selected from styrene and styrenes alkylated in the nucleus and B is a polymer block of one or more partially or completely hydrogenated diene hydrocarbons, and wherein the weight ratio of the polymer blocks A to A' is less than or equal to 1:5 and greater than 1:10, the mean molecular weight of the polymer block A is from 2,000 to 20,000, that of the polymer block A' is from 10,000 to 110,000 and that of the polymer block B is not less than 18,000. 
     
     
       2. The block copolymer of claim 1, wherein the mean molecular weight of the polymer block A is from 2,500 to 4,000 and that of the polymer block A' is from 15,000 to 30,000. 
     
     
       3. A molding material having good processibility and obtained from a block copolymer as claimed in claim 1. 
     
     
       4. The block copolymer of claim 1, wherein the weight ratio of polymer blocks A to A' is less than or equal to 1:6 and greater than 1:8. 
     
     
       5. The block copolymer of claim 4, wherein the vinylaromatic hydrocarbon content is from 25 to 35% by weight and the partially or completely hydrogenated diene hydrocarbon content is from 65 to 75% by weight. 
     
     
       6. A block copolymer of the formula A--B--A' comprising by weight: (a) 18 to 40% of a vinylaromatic hydrocarbon, and   (b) 60 to 82% of one or more hydrogenated diene hydrocarbons, wherein:   (i) A and A' are polymer blocks of vinyl aromatic hydrocarbons,   (ii) B is a polymer block of a hydrogenated diene hydrocarbon that has a degree of hydrogenation of more than 50%,   (iii) the weight ratio of the A/A' polymer blocks is less than or equal to 1/5 and greater than 1/10, and   (iv) polymer block A has a mean molecular weight from 2,000 to 20,000, polymer block A' has a mean molecular weight from 10,000 to 110,000, and polymer block B has a mean molecular weight not less than 18,000.   
     
     
       7. The block copolymer of claim 6, wherein the vinylaromatic hydrocarbon is selected from the group comprising styrene, α-methylstyrene and styrenes alkylated in the nucleus, and wherein the mean molecular weight of polymer blocks A and A' are from 2,500 to 4,000 and from 15,000 to 30,000 respectively. 
     
     
       8. The block copolymer of claim 6, wherein the weight ratio of polymer blocks A to A' is less than or equal to 1/6 and greater than 1/8. 
     
     
       9. The block copolymer of claim 6, wherein the vinylaromatic hydrocarbon content is from 25 to 35% by weight and the hydrogenated diene hydrocarbon content is from 65 to 75% by weight.